The present invention relates to a flotation dryer unit, described in the introduction of the enclosed independent claims, for drying a web, such as a coated paper web, and to a method for feeding drying gas or air towards the web to be dried in the flotation dryer unit.
The present invention especially relates to such flotation dryer units in which drying air is blown onto the web from air distribution channels, extending over the web and combined with supply air distribution chambers arranged on the side of the web.
Various kinds of flotation dryers or cylinder groups, lately also infra-dryers, are used for drying coated paper. The various drying methods have different advantageous features. The choice of dryer and the layout of drying may have an effect, for example, on the quality of the coated paper, energy costs, runnability of the paper web, layout of the coating machine in general (web path draw, space consumption etc.), and investment costs. At present, also combinations of various dryer types are thus used for achieving the optimum drying result, whereby the aim is to combine good properties of the various types.
In a flotation dryer, the evaporation of water from the web is typically achieved by hot air blown onto the web surface from nozzles. The specific evaporation of the flotation dryer is then principally dependent on the temperature and velocity of air blown and also on the type of nozzle used. Commonly used nozzle types comprise, for example, over-pressure and underpressure nozzles, and also direct impingement and orifice nozzles.
Flotation dryers include the following good features: good runnability of paper; good energy economy; minimization of thermal stress in the machine room; double-sided drying possible; controlled drying as the impingement air velocities for the upper and lower sides may be adjusted separately; low risk of overdrying; and further, the structure of the dryer is long-standing and durable. However, the traditional flotation dryer has negative features compared with, for example, an infra-dryer: relatively low specific evaporation; relatively slow warming up of the web e.g. after a break; poorer control properties, i.e. poorer profiling; a lot space consuming; and relatively high investment costs.
In the last few years, attempts have been made to increase the evaporation efficiency of the flotation dryers in the papermaking industry by increasing the blow velocity for drying air from the typical velocity of 40 m/s to a velocity of 60 m/s, even to 80 m/s. Also the drying air temperature has been raised. However, increasing the blow velocity and raising the temperature also increase the size of the dryer, and thus the investment costs and space consumption. These changes also increase the energy consumption and they often have an impairing effect on the controllability.
The air distribution system arranged in the so-called air flotation dryer box of a traditionally constructed flotation dryer is a three-stage system, comprising superimposed air distribution channels both transverse and parallel to the web path, and nozzle channels transverse to the web path.
As blow velocities are increased, also the air volumes blown and, thus, the height of the air flotation dryer box become larger. In this case, the height of the air flotation dryer box typically is already about 1.60 m, with e.g. a web path width of 8.0 m and a blow air velocity of 60 m/s, the efficient length of the flotation dryer in the machine direction being about 2.4 m. Should one still wish to increase the blow velocity, the height of the air flotation dryer box would have to be increased still considerably more in order to keep the flow rates of air in the three-stage air distribution system within the limits permitted by the design, which might, in many cases, cause insurmountable difficulties.
The space consumed by the large height of the air flotation dryer boxes makes it more difficult and, sometimes even impossible, to fit the flotation dryer in connection with a coating machine. Today, the space consumption is to a very large extent also a matter of cost; the smaller the space the dryers may be fitted into, the better.
Thus, it has been proposed, as is apparent, for example, from the American patent publications U.S. Pat. No. 3,964,656; U.S. Pat. No. 4,021,931; and U.S. Pat. No. 4,719,708, that various web floating means with a two-stage air distribution system be used, in which the air is led to nozzle channels travelling across the web path directly from the side of the web path. Thus the size and height of the air flotation dryer box may be kept considerably smaller.
The problem with these arrangements provided with a two-stage air distribution system is, however, how to distribute the impingement air onto the web in an even and controlled way and how to discharge return air evenly from the web. Upon discharging from the nozzles onto the web, drying air flowing over the web with a high velocity causes vibrations and flutter in the web and tends to move the web in the lateral direction. These kinds of movements disadvantageous for the runnability are generated in the web especially in high speed machines or when drying light grade of papers. Also uneven discharge of air from the web area generates unwanted air flows and pressure variations in the vicinity of the web.